Metal-assisted etch combined with regularizing etch

ABSTRACT

In an aspect of the disclosure, a process for forming nanostructuring on a silicon-containing substrate is provided. The process comprises (a) performing metal-assisted chemical etching on the substrate, (b) performing a clean, including partial or total removal of the metal used to assist the chemical etch, and (c) performing an isotropic or substantially isotropic chemical etch subsequently to the metal-assisted chemical etch of step (a). In an alternative aspect of the disclosure, the process comprises (a) performing metal-assisted chemical etching on the substrate, (b) cleaning the substrate, including removal of some or all of the assisting metal, and (c) performing a chemical etch which results in regularized openings in the silicon substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division under 35 USC § 121 of U.S. patentapplication Ser. No. 14/917,698, titled “METAL-ASSISTED ETCH COMBINEDWITH REGULARIZING ETCH,” filed Mar. 9, 2016, which is a U.S. NationalStage Application under 35 U.S.C. § 371 of International Application No.PCT/US2014/053000, filed Aug. 27, 2014, which claims priority to U.S.Provisional Application Ser. No. 61/876,133, filed Sep. 10, 2013, eachof which is incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Contract No.DE-EE0005323 (BA) awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

For some years it has been believed that nanostructuring in lieu of orin addition to standard surface texturing is a road to more efficientbut still cost-effective photovoltaic cells. One way in whichnanostructuring can lead to more efficient photovoltaic cells is byreducing reflection of light out of the cell, producing what has oftenbeen referred to as “black silicon.” Light that is reflected away fromthe sun-facing surface of the photovoltaic cell is lost in the sensethat its energy cannot be converted to electricity by the photovoltaiccell.

A wide range of nanostructuring has been proposed. The present assigneehas studied the formation of nanowires by etching as a road tonanostructuring, as discussed for example in U.S. Published PatentApplication No. 2009/256134, now U.S. Pat. No. 8,143,143. Specifically,the use of metal-assisted etching of silicon has been widely regarded aspromising.

The beneficial effects of nanostructuring are seen in the literature andin experience to be achievable with a wide range of specific shapes. Acommon denominator in some proposals is that the structures havegeometric dimensions roughly of the order of the wavelength of the lightwhich produces the electrical energy.

There is still a need for nanostructuring and cost-effective techniquesto form it which lead to a net higher efficiency and other beneficialcharacteristics for photovoltaic cells.

SUMMARY OF THE INVENTION

In an aspect of the disclosure, a process for forming nanostructuring ona silicon-containing substrate is provided. The process comprises (a)performing metal-assisted chemical etching on the substrate, (b)performing a clean, including partial or total removal of the metal usedto assist the chemical etch, and (c) performing an isotropic orsubstantially isotropic chemical etch subsequently to the metal-assistedchemical etch of step (a).

In accordance with another aspect, there is provided asilicon-containing substrate having a top surface which comprisesnanostructuring which comprises substantially rounded depressions andwhich comprises residual metal deposits in at least some of thesubstantially rounded depressions. The residual metal deposits maycomprise at least about 10¹⁰ atoms/cm², 10¹¹ atoms/cm², or 10¹²atoms/cm².

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts nanoparticles of silver on a substrate surface formedduring a process similar to that of Exemplary Process 1.

FIG. 2 depicts a top view of the nanostructuring formed as a result ofExemplary Process 1.

FIG. 3 depicts a side view of the nanostructuring formed as a result ofExemplary Process 1.

FIG. 4 depicts a top view of the nanostructuring formed as a result ofExemplary Process 2.

FIG. 5 depicts a side view of the nanostructuring formed as a result ofExemplary Process 2.

FIG. 6 depicts a top view of the nanostructuring formed as a result ofExemplary Process 3.

FIG. 7 depicts a side view of the nanostructuring formed as a result ofExemplary Process 3.

FIG. 8 depicts reflection as a function of nanostructure length in threemanufacturing processes, where Process 1 is a process of the disclosure,whereas Processes 2 and 3 are processes of the kind disclosed inPCT/US14/13677, filed Jan. 29, 2014, assigned to the present assignee.

DETAILED DESCRIPTION OF THE INVENTION

Before describing the present invention in detail, it is to beunderstood that this invention is not limited to specific solvents,materials, or device structures, as such may vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting.

Where a range of values is provided, it is intended that eachintervening value between the upper and lower limit of that range andany other stated or intervening value in that stated range isencompassed within the disclosure. For example, if a range of 1 μm to 8μm is stated, it is intended that 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, and 7 μmare also disclosed, as well as the range of values greater than or equalto 1 μm and the range of values less than or equal to 8 μm.

In an aspect of the disclosure, a process for forming nanostructuring ona silicon-containing substrate is provided. The process comprises (a)performing metal-assisted chemical etching on the substrate, (b)performing a clean, including partial or total removal of the metal usedto assist the chemical etch, and (c) performing an isotropic orsubstantially isotropic chemical etch subsequently to the metal-assistedchemical etch of step (a). In an alternative aspect of the disclosure,the process comprises (a) performing metal-assisted chemical etching onthe substrate, (b) cleaning the substrate, including removal of some orall of the assisting metal, and (c) performing a chemical etch whichresults in regularized openings in the silicon substrate.

Step (a), the process of metal-assisted chemical etching, may beperformed by a variety of steps which have been explored over the pastdecade, including by the present assignee. Step (b), the cleaning of thesubstrate, may take place by a variety of cleans widely accepted inindustry, including cleans for removal of the metal used to assist thechemical etch. The regularizing chemical etch of step (c) may be one ofa variety of chemical etches. Such chemical etches may be isotropic orapproximately or substantially isotropic. As part of regularization, theroughly 90 degree angle edges often left by the metal-assisted chemicaletching may become more rounded. It is also possible to produce somedegree of regularization with an anisotropic etch, provided it is nottoo similar in effect to the metal-assisted chemical etch.

In an exemplary process for step (a), the assisting metal is laid in apattern over the surface of a silicon or silicon-containing substrate.The assisting metal is preferably a noble metal, for example silver orgold. The pattern is preferably self-assembled rather than beinggenerated from a mask or other similar template. A way to lay thispattern is to deposit a fairly thin layer, on the order of 8-20 nm, forexample by sputtering, in such a way that the deposited layer of metalagglomerates as it is being deposited, forming islands, also morecolloquially called blobs. Alternatively, deposition may be achieved,for example, from solution using, for example, AgNO₃ to deposit silver.The islands may then be subject to a process that further fostersagglomeration, for example an annealing process. The islands may also beetched in order to shrink them. The islands may be observed readilyunder SEM, as shown for example in FIG. 1. In other metal-assistedetching processes, it is possible to lay down more elaborate structuresnot necessarily consisting of islands. An example of such a structure isa set of islands plus a covering of deposited metal which occupies themajority of the area between the islands, wherein there are gaps betweenthe blobs and the latter covering.

A variety of treatments may be employed for the step of agglomeratingthe deposited assisting metal. The agglomeration step may be carriedout, for example, by heating the substrate with the initially depositedmetal on it. The heating may take place in a chamber used fordeposition, for example in the sputtering tool if the first metaldeposition was through sputtering. The heating may alternatively takeplace in a separate chamber or oven. The heating or other agglomerationmay take place in a liquid solution, for example water. The heating maybe carried out, for example, at a temperature of at least about 150° C.,at least about 200° C., at least about 250° C., at least about 300° C.,at least about 350° C., or at least about 400° C. The heating may becarried out, for example, for a time on the order of minutes, forexample between about 1 and 30 minutes, about 2 and 10 minutes, or about3 and 7 minutes.

In the processes as described in this application, no use need be madeof patterning by means of photolithography. While additionalphotolithographic steps are not excluded, it is believed that theprocess of agglomeration as described forms adequate nanosized patternsin many circumstances. The latter patterns are often suitable formetal-enhanced etching of useful nanostructures, providing adequatecontrol for many applications without the need for the expense ofphotolithography.

Once the assisting metal has been deposited in a suitable pattern, itbecomes possible to carry out the etch using a suitable etchant, forexample HF with a suitable oxidizer, which may be for example HF withH₂O₂ or HF with oxygen bubbled through it. The metal assisted etchcauses etching where the metal is present and results in an approximatedrilling down into the silicon. It is therefore not isotropic but ratheroperates in a direction roughly normal to the surface. The directiontends to be closer to precisely normal where the surface being etched isof (100) crystalline orientation versus where it is of other crystallineorientations as is seen, for example, in multicrystalline silicon inwhich each grain may have its own orientation. The depth of the etch isin many conditions readily controllable. The aggressiveness of the etchmay be varied, for example, by varying the strength of the oxidant. Theduration of etching may be varied, for example, to vary the dimensionsof the nanostructuring produced, and can vary for example between about1 to about 15 minutes. Following the etching, one may in some cases drythe etched substrate, for example by drying the substrate in a spinrinse dryer after rinsing.

With metal-assisted etching as the nanostructure-producing etch, it hasbeen possible to achieve a number of types of nanostructuring which makeuseful contributions to the efficiency and other desirablecharacteristics of photovoltaic cells. Because metal-assisting etch isnot isotropic, for example, it can produce quite tall nanostructures.

However, it has surprisingly been discovered that a combination ofmetal-assisted etching with a subsequent regularizing etch, as in step(c) above, can be of value in achieving desirable values of theparameters pursued for photovoltaic applications, such as reflectance,quantum efficiency, and short circuit current. Specific recipes for theuse of a regularizing etch in combination with a metal-assisted etch aregiven below.

It is possible to see the structure formed during the metal enhancedetch as being a template used to form the nanostructuring on the siliconsurface using the regularizing etch.

Prior or subsequent to performing the regularizing etch, one may subjectthe substrate to a cleaning step. The cleaning step is preferably onewhich removes the etch-assisting metal in whole or in part. For example,a piranha clean may be used.

Nothing about the present disclosure requires that the etch of step (c)be precisely isotropic. It may also be approximately or substantiallyisotropic or, indeed, anisotropic if capable of having a desirableregularizing effect. Indeed, while the etch used in the examples belowis generally regarded as isotropic, it may not be fully isotropic in thecontext of the nanostructures being etched. Furthermore, for a desirablerounded morphology and/or for the bottom line goals of reflectivity,quantum efficiency, etc., for photovoltaics or related goals for otherdevices, an anisotropic etch may be adequate if it is not aspredominantly straight-down as a metal-assisted etch. Indeed, some metalmay remain as residue despite the cleaning of the substrate. Theresidual metal may be observed, for example, via SEMs, or withinductively coupled plasma mass spectrometry (ICPMS), or with surfacesecondary ion mass spectroscopy (SIMS). In the event of some residueremaining or in the event the metal is cleaned only after the secondetch, the second etch may involve a chemical-assisted component if asuitable etchant is used.

We do not wish to be bound by any particular theory of why furtheretches after metal-assisted etches can achieve desirable performance.However, we believe that the regularizing etch can remove surfacedefects while leaving a nanostructuring of desirable dimensions. Forexample, in certain experiments forming nanowires by metal-assistedchemical etching, porous silicon is observed on the tips of thenanowires, and it may be desired to remove it. The surface defects mayalso encompass, for example, recombination centers which would tend toreduce the performance of a photovoltaic cell by increasing the rate ofelectron-hole recombination.

In addition to what is stated above, one seemingly desirable result of aregularizing etch is the roundedness of the etched-away volumespecifically towards its bottom. This roundedness is seen, for example,in FIGS. 3, 5, and 7 of this application. Based on a crude “ray tracing”understanding of the light which impinges on a photovoltaic cell, itappears that light can in some circumstances be better captured andprevented from reflecting away from the cell by a rounded structurewhose dimensions are of the approximate order of magnitude of thewavelength. One visualizes, for example, rays which would have hit abottom with rounded edges close to vertically being reflected back up,whereas the same rays hitting a rounded bottom portion of a depressionbeing reflected sideways and so impinging on the walls surrounding thedepression and thus having a greater chance of ultimately being absorbedand being potentially available to generate electrical energy (or, innon-photovoltaic devices, for example, a detectable signal).

The second etch may have a further advantage in certain processes formanufacturing solar cells, in that it permits the step of saw damageremoval to be eliminated or abbreviated. The overall processes of thisdisclosure with two etching steps may also produce results as effectiveor more effective than the reactive ion etches which have been proposedfor solar cell manufacturing.

More generally, the type of process described in this disclosure maylead to solar cells with nanostructuring to which screen printedcontacts are better (e.g., have lower contact resistance). Solar cellswith such nanostructuring may, for example, have a better fill factor.In addition or alternatively, if better contacts are possible as aresult of the nanostructuring, one may choose to decrease the surfacedoping concentration below that of standard processes. The lower thesurface doping concentrations, the higher the contact resistance, butone may with such doping obtain better blue response and betterpassivation leading to better open-circuit voltages for the solar cells.The better contacts may also make it possible to employ doping processeswhich are less uniform across the wafer, such as doping with low POCl₃rates.

In many cases, the two-step etch of the disclosure can be visualizedroughly as first creating depressions in the silicon-containingsubstrate, which are then expanded outward by the regularizing etch.When the latter etch is isotropic or substantially isotropic, the depthof the depressions will persist initially during the latter etch becauseit etches away the top of the silicon-containing substrate as well asthe bottoms of the depressions at roughly the same nm/s rate, but thedepressions become wider and more rounded.

With the types of pattern formation described above for the assistingmetal, the location of the islands or “blobs” will have a randomcomponent. This suggests that the depressions, as they are expanded bythe regularizing etch, may come to overlap with each other. The overlapwill vary considerably, for example, according to how much etch isemployed and according to the average distance between depressions andthe precise probability distribution of these distances. The overlapwill leave nanostructuring of different appearances, which may forexample comprise nanowires sticking up (see for example FIG. 5) orcurved walls somewhat reminiscent of the curved walls found in certaingardens (see for example FIG. 7).

The rounded depressions of this disclosure may be formed at a variety ofdepths, for example depths in excess of about 1 μm, 500 nm, 100 nm, 50nm, or 20 nm. They may also be formed at a variety of densities, forexample at least about 10 depressions per μm², at least about 20depressions per μm², at least about 30 depressions per μm², at leastabout 60 depressions per μm², or at least about 100 depressions per μm².They may have a variety of ranges of overlap, with the overlappingvolume being at least about 5%, 10%, 20%, 40%, or 60% of the totalvolume of the depressions.

A reduction in reflectivity may result from the fact that as oneapproaches the substrate, the percentage of silicon versus othermaterial (e.g., air, SiN_(x)) in a plane parallel to the substrateincreases. For example, when one is above the nanostructuring, thepercentage of silicon in such a plane parallel to the substrate is zero.The percentage then rises above zero as one reaches the top level of thenanostructuring. The percentage keeps increasing as one approaches thesubstrate. Then, when one reaches the bottommost of all the depressions,the percentage of silicon reaches its full bulk value (e.g., 100% forpure silicon). As a result of the increase of the percentage of siliconas one approaches the substrate in a direction perpendicular to thesubstrate, the effective index of refraction may also be expected tovary as one approaches the substrate in such a direction. The percentageof silicon averaged over the transition period is a measure of thedensity of the nanostructuring. It may be, for example, less than about5%, less than about 10%, less than about 25%, or less than about 60%; itmay be, for example, greater than about 5%, greater than about 10%,greater than about 25%, or greater than about 60%. The percentage ofsilicon at the average nanostructure height is an alternative measure ofthe nanowire density. It may be, for example, less than about 5%, lessthan about 10%, less than about 25%, or less than about 40%; it may be,for example, greater than about 5%, greater than about 10%, greater thanabout 25%, or greater than about 40%.

FIG. 8 depicts reflection as a function of nanostructure length in threemanufacturing processes, where Process 1 is a process of the disclosure,whereas Processes 2 and 3 are processes of the kind disclosed inPCT/US14/13677, filed Jan. 29, 2014, assigned to the present assignee.The latter processes differ in that Process 2 has 80 nm of alumina, asdoes Process 1, whereas Process 3 has 25 nm alumina. As may be seen, forrelatively short nanowires a lower reflection is achieved withProcess 1. A solar cell made with processes of the disclosure may have areflectance of no more than about 2%, 3%, or 4%, with nanowires whichhave a length of no more than about 0.4 nm, 0.5 nm, or 0.6 nm.

While the primary application which motivated the inventors in studyingthe etches of this disclosure was photovoltaic, there is no reason whythe type of nanostructuring achieved in this disclosure would not beapplicable in other types of devices which are created withnanostructuring. For example, the enhanced efficiency of capture ofincoming light may be useful in a variety of detection scenarios. Thestructures might also be useful for LEDs, field emission devices, orsubstrates for Raman spectroscopy.

The silicon-containing substrate referred to in this disclosure maycomprise monocrystalline, polycrystalline, or amorphous silicon. Theentire substrate may consist of silicon (plus dopants, an inevitablesmall layer of “native” SiO₂ at the surfaces, and other impurities). Thesilicon may be alternatively layered (for example deposited) on adifferent material. Techniques like the present ones may also apply, forexample, to substrates comprising Si/Ge materials.

Photovoltaic processing of the nanostructured silicon-containingsubstrates of the present disclosure involves a series of additionalsteps. These may include, for example, the formation of a p-n junction,edge isolation, passivation, screen printing of the front-side contacts,screen printing of the rear contacts, and firing of the contacts.

A particularly significant step in the formation of a photovoltaic cellis the formation of a p-n junction. Such a junction may be formed in thesilicon nanostructuring or in the substrate below the siliconnanostructuring. The junction may be formed by a variety of dopingtechnologies known to those of skill in the art, for example POCl₃diffusion or ion implantation. In some cases a p-i-n junction may beemployed.

The photovoltaic application may also require contacts to be made to thefront and back sides of the silicon-containing substrate after formationof the p-n junction. A variety of approaches may be taken to thosecontacts, as described for example in U.S. Published Patent ApplicationNo. 2013/0099345 to the present assignee, now U.S. Pat. No. 8,852,981.

In what follows, exemplary processes of the disclosure are described.The following examples are put forth so as to provide those of ordinaryskill in the art with a more complete disclosure and description of howto implement the invention, and are not intended to limit the scope ofwhat the inventors regard as their invention. Efforts have been made toensure accuracy with respect to numbers (e.g., amounts, temperature,etc.) but some errors and deviations should be accounted for.

Exemplary Process 1

Monocrystalline silicon material with a resistivity of 1-3 ohm-cm isselected having a surface with a (100) crystallographic orientation andp-type doping.

The first step is to do a saw damage removal process on the wafers. Thesaw damage removal comprises precleans and a KOH etch that removes thedamage caused by forming the wafers. This is performed by first cleaningthe wafers in a Piranha solution for 10 minutes. The Piranha step ofthis exemplary process employs 4 parts 96% sulfuric acid (H₂SO₄) and 1part 30% hydrogen peroxide (H₂O₂) at elevated temperature. Prior toplacing the wafer into the piranha solution, the latter is bubbled withN₂ for 2 minutes. The bubbling is then stopped and the wafer issubmerged for 10 minutes. The solution is heated from the exothermicreaction. After the piranha etch, the wafer is cleaned 3 times with DI(deionized) water.

Next the wafers are dipped for 1 minute in a mix of 1 part 49% HF to 4parts DI water to etch off the oxide formed during the Piranha clean.After the HF clean the wafers are removed and again rinsed three timesand placed into a spin rinse dryer.

Next a polish etch is performed for 10 minutes in a solution of 1 partDI water, 1 part 45% potassium hydroxide (KOH) at about 70° C. Thewafers are then rinsed three times in DI water. The wafers are thencleaned again with Piranha for 10 minutes and rinsed with water threetimes. The wafers are then cleaned in HF and again rinsed three timeswith water.

The sample is placed into a sputtering chamber for the deposition of thesilver layer. The base pressure in the chamber is pumped down to7.0×10⁻⁷ Torr and then 14 nm of silver (Ag) is sputter deposited on thefront of the wafer at a rate of about 7 Å/s at a pressure of 5 mTorr.The thickness of the silver is determined by a crystal monitor. Thesamples are then removed from the chamber.

The next step is to heat the layer of silver so that it coalesces andagglomerates on the surface forming into ball-like nanoparticlestructures distributed more-or-less evenly on the surface of the wafer.The heating process can be done in situ in the sputtering tool or thesample can be removed from the sputtering tool and annealed in an ovenor a furnace. In this example, the samples were removed from thesputtering tool and placed in a box furnace. The samples were thenheated in a metal cassette at 300° C. for 5 minutes. The shape of thesilver nanoparticles may be seen in FIG. 1, which is taken from asimilar processing step in which about 18.5 nm of silver was deposited.

After forming nanoparticles of silver, they can then be shrunk toachieve a desired size. In the exemplary process, the nanoparticles areplaced in an etch in 8000 mL nitric acid and 160 mL HF for 6 seconds.

The next step is an etch which uses the silver on the surface of thesilicon to form a nanostructured silicon surface. The nanostructureformation occurs in an oxygen/HF bath. The wafer is placed in a dilutehydrofluoric acid (HF) bath. The bath contains 10 parts volume water to1 part volume HF. The sample is etched in the HF bath for 6 minutesduring which time oxygen is bubbled vigorously through the HF using aperforated Teflon tube (or alternatively a polypropylene tube). Afterthat, the sample is rinsed three times in DI.

The silver is then removed and the sample is cleaned in a series of wetbaths. The first bath is a piranha clean which consists of 4 ml ofsulfuric acid (H₂SO₄) to 1 ml of 49 wt % hydrogen peroxide (H₂O₂) at anelevated temperature around 70° C. Prior to placing the wafer into thepiranha, the bath is bubbled for 2 minutes. The wafer is submerged for 2minutes. After the piranha etch, the wafer is cleaned 3 times with DIwater.

The sample is then placed in dilute HF for 30 seconds. This HF solutionhas a volume ratio of 10:1 of water to 49% HF and is at roomtemperature. The wafer is then rinsed three times with DI water anddried in the spin rinse dryer.

The nanostructured wafer is then etched again using 8000 mL nitric acidplus 160 mL HF for 60 seconds. The wafer is then rinsed three times withDI water and dried in the spin rinse dryer.

Exemplary Process 2

In a variation of the preceding process, the step of shrinking thesilver nanoparticles is omitted, and the starting material is amulticrystalline wafer. The result may be seen in FIGS. 4 (top view) and5 (side view).

Exemplary Process 3

This exemplary process is the similar to exemplary process 2, except fortwo changes. The first is that instead of a 6 minute etch in HF bubbledwith oxygen, only a 3 minute etch was carried out. The second differenceis instead of a 60 second etch in 8000 mL nitric acid plus 160 mL HF,the etch was carried out for 15 seconds. The result may be seen in FIGS.6 (top view) and 7 (side view).

The following references may be relevant to this application: (1) SamiFranssila, Introduction to Microfabrication (2d ed. John Wiley & Sons2010). (2) U.S. Published Patent Application No. 2009/256134 to thepresent assignee. (3) X. Li and P. Bohn, Appl. Phys. Letters, vol. 77,pp. 2572-2574 (2000). (4) Hidetaka Asoh et al., Electrochem. Comm. vol.9, pp. 535-539 (2007). (5) Jianwei Shi et al., Solid State Electronics,vol. 85, pp. 23-27 (2013). (6) Dirk-Holger Neuhaus and Adolf Münzer,Advances in OptoElectronics, Volume 2007, Article ID 24521. (7) AlexStavrides et al., “Plasma Texturing for Efficiency Improvement in c-SiSolar Cells,” 6th SNEC PV Power Expo, 6-8 May 2012, Shanghai. (8)Jianhua Zhao et al., “A 19.8% Efficient Honeycomb MulticrystallineSilicon Solar Cell with Improved Light Trapping,” IEEE Transactions onElectron Devices, Vol. 46, pp. 1978-83 (1999). (9) U.S. Published PatentApplication No. 2007/62575 to Kyocera.

All patents, patent applications, and publications mentioned in thisapplication are hereby incorporated by reference in their entireties.However, where a patent, patent application, or publication containingexpress definitions (including definitions via disclaimer) isincorporated by reference, those express definitions should beunderstood to apply to the incorporated patent, patent application, orpublication in which they are found, and not to the remainder of thetext of this application, in particular the claims of this application.

The invention claimed is:
 1. A process for forming nanostructuring on asilicon-containing substrate, the process comprising: (a) performingmetal-assisted chemical etching on the substrate to form nanostructureddepressions extending downward from a surface of the substrate; (b)performing a clean subsequently to the metal-assisted chemical etch ofstep (a), the clean including removal of substantially all of the metalused to assist the chemical etch; and (c) performing an isotropic orsubstantially isotropic chemical etch subsequently to the clean of step(b), the isotropic or substantially isotropic chemical etch etching thesurface the substrate and bottoms of the depressions at approximately asame rate.
 2. The process of claim 1, wherein the isotropic orsubstantially isotropic etch of step (c) comprises immersion in anetchant solution comprising HNO₃ and HF.
 3. The process of claim 2,wherein the etchant solution further comprises acetic acid.
 4. Theprocess of claim 1, wherein the etch of step (a) removes between about0.3 and about 3 times as much material, measured in nm, as the etch ofstep (c).
 5. The process of claim 1, wherein the step (a) comprisesdepositing the metal which assists the etch and subjecting the depositedmetal to a process of agglomeration.
 6. The process of claim 5, whereinthe step (a) comprises etching the agglomerated metal.
 7. The process ofclaim 5, wherein the process of agglomeration comprises annealing. 8.The process of claim 5, wherein the process of agglomeration isperformed in a liquid solution.
 9. The process of claim 1, wherein theresult of (c) is further processed to form an optoelectronic orphotovoltaic device with nanostructuring.
 10. The process of claim 9,wherein the device formed is a solar cell.
 11. The process of claim 10,wherein the solar cell formed exceeds the efficiency of a solar cellwith otherwise identical processing in which nanostructuring is formedby means of metal-assisted chemical etching not followed by a furtherisotropic or substantially isotropic chemical etch.
 12. The process ofclaim 10, wherein the solar cell formed exceeds the efficiency of asolar cell with otherwise identical processing in which nanostructuringis formed by means of reactive ion etching.
 13. The process of claim 10,wherein the solar cell is not subject to a step of removing saw damage.14. The process of claim 1, wherein the etching removes material in sucha way that the result of step (c) comprises nanowires.
 15. The processof claim 1, wherein the metal-assisted chemical etching of step (a) isan anisotropic etch.
 16. A process for forming nanostructuring on asilicon-containing substrate, the process comprising: (a) performingmetal-assisted chemical etching on the substrate to form nanostructureddepressions extending downward from a surface of the substrate; (b)performing a clean of the substrate and nanostructured depressionsformed in step (a) subsequently to the metal-assisted chemical etch ofstep (a) to form a cleaned substrate, the clean including removal ofsubstantially all of the metal used to assist the chemical etch; and (c)performing an isotropic or substantially isotropic chemical etch of thecleaned substrate formed in act (b) subsequently to the clean of step(b), the isotropic or substantially isotropic chemical etch etching thesurface the substrate and bottoms of the depressions at approximately asame rate.
 17. The process of claim 16, wherein the metal-assistedchemical etching of step (a) is an anisotropic etch.